The falling out began Monday, when Nagoya’s mayor, Takashi Kawamura, told a visiting delegation of Chinese Communist Party officials from Nanjing that he doubted that Japanese troops had massacred Chinese civilians. Most historians say that at a minimum, tens of thousands of civilians were slaughtered in Nanjing in one of the most infamous atrocities of Japan’s military expansion across Asia in the early 20th century.

The falling out underscored how differing views of history remain a problem in Japan’s ties with the nations that it once conquered. While such denials are common by Japanese conservatives like Mr. Kawamura, they are rarely raised in such a public manner, or directly to Chinese officials. But there is also a widely shared perception in Japan that China’s government plays up the massacre for its own propaganda purposes ….

On Wednesday, Mr. Kawamura remained unrepentant, saying that he did not intend to retract the statement or apologize. He explained that his father had been a solider in Nanjing in 1945, and was treated kindly by city residents, which he said would have been impossible had an atrocity taken place there just eight years earlier.

“Nanjing should invite Kawamura Takashi to tour the Nanjing Massacre Memorial Hall,” one user wrote on popular Twitter-like microblogging service Sina Weibo, where Mr. Kawamura was among the most-discussed topics on Wednesday.

Others, however, directed their ire at Liu Zhiwei, the head of the Nanjing delegation, after Kyodo reported that Mr. Liu shook hands with Mr. Kawamura and didn’t directly challenge his denial of an event often described as Asia’s equivalent to the Holocaust.

“All the ghosts of the Nanjing Massacre are going to come knocking on Liu Zhiwei’s door,” wrote one Weibo user.

A Global Times editorial also focused on the Nanjing delegates’ allegedly ineffectual response. Editor-in-chief Hu Xijin shared it on Twitter:

Chinese officials should also believe that in the diplomatic world, there is no fuss that is too big to be caused. It is not a strategy, but a courageous style when China is squeezed on many diplomatic fronts. A relationship with a particular country getting a little better or worse would not impede China’s development. This attitude can prevent major events from erupting from small matters.

Diplomacy means honestly showing what we truly care about. It does not interfere with diplomatic skill.

While attending to those of others, we should also attend to our own feelings. It is a pity that almost all frictions in China’s diplomatic exchanges were first broken by foreign sides. Chinese officials should learn how to use the media to tilt public opinion against the provoker.

We advise China to levy sanctions on Kawamura, for example listing him as an unwelcome person and barring his entry into China. Nagoya can be delisted from the schedule of Chinese tourism groups to Japan. China can also consider reducing economic exchanges with the city.

These are fully reasonable steps. Kawamura directly offended the delegation from Nanjing, the city victimized by the brutal killings in 1937. It is a serious mistake both from a diplomatic and historical perspective. As a result, he has infuriated the whole of Chinese society. Due punishment will appease the Chinese public, which has long thought of the Chinese diplomatic approach as weak ….

Punishing Kawamura is right. We understand the questions raised by a few Japanese rightists on Nanjing Massacre under certain circumstances. But Kawamura, as a politician, has crossed the line. A similar mistake would cost dearly for politicians everywhere. Imagine if a Chinese official applauded the atom bombs dropped on Japan in front of a delegation from Hiroshima or Nagasaki. Could the Japanese accept it?